Screw cap of thermoplastics material

Abstract

A screw cap of thermoplastic material, particularly intended for the closure of glass containers used in the food industry, is provided on the outer surface of its horizontal end wall (1) with a thin covering layer (13) of a composite laminated material which is adapted to form a barrier against the passage of gases through the wall. This composite, laminated material is constituted by a disc (13a) incorporated in the outer face of the horizontal end wall (1) of the cap during the injection moulding of the cap. The disc (13a) is permanently bonded to the horizontal end wall (1) of the cap and to a downwardly-curved wall (10) connecting the peripheral edge of the horizontal end wall (1) to a rounded annular shoulder (11) provided at the top of the cylindrical skirt (2) of the cap. The cap is made by a method in which the disc (13a) is inserted into the cavity of the matrix (19) of an injection mould (17, 18) and is held in place by frictional engagement of its peripheral edge with the ribs (23) of said matrix (19) which are intended to form vertical grooves (12) in the cylindrical skirt (2) of the cap.

What is claimed is: 
     
       1. A method for the manufacture of a screw cap of thermoplastic material having a horizontal end wall, including a downwardly-curved wall defining a peripheral edge of said horizontal end wall, a cylindrical skirt with internal threads and spaced apart external vertical grooves and a thin covering layer of a composite laminated material on an outer surface of the horizontal end wall to form a barrier against the passage of gases through the wall using a mold comprised of a matrix and a core which are capable of relative movement between an open and closed position, the method comprising: positioning a disk which will form said thin covering layer in said matrix with the outer periphery of the disk fictionally engaged with projections on said matrix adapted to form the grooves in said skirt;   closing the mold to bring an end surface of the core into close proximity to said disk; and   injecting thermoplastic material through said core into a space between the core and the matrix so as to form the cap and simultaneously incorporate the disk in the horizontal end wall while simultaneously pressing the peripheral edge portion of the disk into an annular groove in the matrix whereby the outer peripheral edge of the disk will overlie the horizontal end wall of the cap and the downwardly-curved wall defining the peripheral edge of the horizontal end wall.